1. Field of the Invention
This invention is related to buoys, rafts and aquatic devices and more particularly to a floatation collar for an undersea acoustic receiver and method of positioning a plurality of the same in the presence of fishing, gear or other activities that would potentially disrupt the sensor positioning within the water column.
2. Description of the Prior Art
Acoustic receivers are used in many underwater applications such as identification of sub-surface vessels, marine mammals and fish. These receivers are expensive and sensitive electronic devices and can be rendered inoperative if exposed transducers are damaged. The challenges in placing these devices in an underwater environment relate to (1) protection the device from commercial fishing gear (such as trawls or long-lines) in such a way that its operation is not impeded, (2) deployment of the device occurs in a fast and cost-efficient manner while at sea, and (3) recovery of the device for repair and re-use is Meditated.
Typically, an acoustic receiver is encased in buoy-like objects that protect the device and enables recovery. The receiver is generally tethered to either a surface float or sub-surface floats located above the receiver that help maintain the acoustic receiver at a desirable predetermined depth. For many purposes, the depth that the receiver must be positioned in the water column is critical because, for example in the field of animal telemetry, acoustic transmitters (“tags”) small enough to be surgically implanted into small fish have low power transmissions in order to conserve the battery. As a result, in locations where the water depth is near to or exceeds the transmission range of the signal, the receivers must be placed in mid-water in order to satisfactorily detect the signals emitted from tagged animals that might be located near either the bottom or the surface. In areas of rough bottom terrain, the receivers also need to be lifted well above the bottom in order to “see” downwards over a wide area. Placing the receivers at or near the sea surface is not feasible because storms and biofouling sharply degrade the operational life of equipment, necessitating their sub-surface placement at depths deep enough to be little affected by storms or biological fouling (which is largely light-dependent). In the case of acoustic telemetry receivers, placing sub-surface floats above the receiver creates a shadow and occludes part of the area that is of interest to monitor, as well as increasing the chance of the system will become fouled and displaced by fishing activities (trawlers or groundlines). For many purposes, degradation of arrays of receivers due to either physical loss or displacement of units or impairment of their detection capability results in substantial economic costs to compensate for reduced performance.
One such object 10 that can address these needs is shown as FIG. 1 and is labelled prior art. The casing 12 is somewhat barrel-shaped and has a number of disadvantages. The circular shape of the barrel is not stable when placed on the boat deck, and the square bottom profile presents a potential point where commercial fishing gear can snag at the junction of the casing and the mooring tether. This may result in the buoy being snagged by fishing gear and lost or moved out of position in an array of geographically positioned sensors. The top projections of the barrel 14 are not sufficiently long to protect the sensitive emitters and detectors on the acoustic device 16 and so they remain prone to mechanical damage. The tether mechanism 18 connecting the buoy 10 to an anchor is metallic and prone to corrosion and mechanical wear by wave action. This may sever the tether 24 which may result in loss of both the weight and the acoustic device.
Therefore, there is a requirement for a positioning system that overcomes the deficiencies noted above.